The present invention pertains to the art of metal surface treatment, and more particularly, to conversion coatings for plated metals.
The invention is particularly suited to treatment or passivation of zinc-plated metal surfaces using a trivalent-chromium conversion coating and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications such as treatment of other types of plated or non-plated substrates.
In the metal finishing industry, conversion coatings are used to provide metal surfaces with improved corrosion resistance. Conversion coatings also provide metal surfaces with improved adhesion for additional coatings such as paint or other finishes. Widely used conversion coatings use hexavalent chromate. Such chromate conversion coatings are applied at various thicknesses, ranging from a very thin “blue-bright” finish to a very thick olive-drab finish. “Blue-bright” finishes are transparent with a slight blue tint and high luster. Such a finish not only imparts a corrosion-resistant coating to the surface of a substrate but also aesthetically enhances the substrate and articles made therefrom. Heavier chromate conversion coatings are considerably more protective than the bright finishes, but they do not meet the aesthetic criteria that are characteristic of the bright coatings. These heavier coatings are well-recognized by their yellow, bronze, or olive-drab finishes which correspond in general order to increasing film thickness.
Although conversion-coating techniques using hexavalent chromium provide satisfactory results, hexavalent chromium can be toxic. Therefore, the waste from a hexavalent chromium based solution creates significant environmental concerns and hexavalent chromium baths require special treatment prior to disposal.
There are non-chrome treatments for forming passivation coatings, but these are generally unsatisfactory for enhancing the corrosion resistance of a plated substrate, particularly when the treated substrate is subjected to a humid environment. Such treatments typically include phosphate treatments and a bright dipping step, followed by a coating step using a transparent lacquer. Each step provides an additional barrier layer to corrosive conditions, but not a cohesive film forming a chemical bond between a film-forming element and the coated substrate as is achieved by hexavalent chromium processes. Accordingly, phosphate treatments provide barriers that are porous, permitting moisture to pass through to the coated substrate. Phosphate coatings are also not bright in appearance, but provide dull, opaque paint-like overcoats lacking the aesthetic attributes of hexavalent chromium conversion coatings.
Hexavalent chromium-free treatments have been developed utilizing trivalent chromium. For example, U.S. Pat. No. 4,349,392 is directed to an aqueous acidic solution and process for treating metal surfaces, comprising chromium ions substantially all of which are in the trivalent state, hydrogen ions to provide a pH of about 1.2 to about 2.5, an oxidizing agent, a bath soluble and compatible organic carboxylic acid or metal salts, and at least one additional metal ion.
U.S. Pat. No. 4,359,345 teaches an aqueous acidic solution and process for treating metal surfaces comprising chromium ions substantially all of which are in the trivalent stet, hydrogen ions to provide a pH of about 1.5 to 2.2, an oxidizing agent, and iron ions in combination with at least one additional metal ion.
U.S. Pat. No. 4,359,346 is directed to an aqueous acidic solution and process for treating receptive metal surfaces containing chromium ions substantially all of which are in the trivalent state, hydrogen ions to provide a pH of about 1.2 to 2.5, an oxidizing agent, and cerium ions.
U.S. Pat. No. 4,359,348 teaches an aqueous acidic solution and process for treating metal surfaces. The solution contains chromium ions substantially all of which are present in the trivalent state, hydrogen ions to provide a pH of about 1.2 to 2.5, an oxidizing agent, a stabilizing agent, and at least one additional metal ion.
U.S. Pat. No. 4,367,099 teaches an aqueous acidic treating solution containing as its essential constituents chromium ions, substantially all of which are present in the trivalent state, hydrogen ions to provide a solution pH of about 1.2 to 2.5, an oxidizing agent, and at least one additional metal ion present in an amount effective to activate the bath and formation of a chromate passivate film of the desired appearance on the substrate treated.
U.S. Pat. No. 4,384,902 teaches an aqueous acidic solution containing chromium ions substantially all of which are in the trivalent state, hydrogen ions to provide a pH of about 1.2 to 2.5, an oxidizing agent, a bath soluble and compatible silicate compound to provide improved corrosion protection, and at least one additional metal ion.
U.S. Pat. No. 4,578,122 teaches an aqueous acidic treating solution comprising chromium ions substantially all of which are present in the trivalent state, hydrogen ions to provide an operating solution pH of about 1.2 to 2.5, nitrate ions as the essential oxidizing agent, and at least one additional activation metal ion. Importantly, the patent require at least 4:1 nitrate ions to chromium and activating metal ions, resulting in excess oxidizing agent which is damaging to the overall film quality and may lead to generation of undesirable Cr(VI).
Finally, U.S. Pat. No. 6,096,140 teaches a metallic surface treating solution with a pH of 0.1 to 6.5 comprising a source of at least one selected from the group consisting of Mo, W, V, Nb, Ta, Ti, Zr, Ce, Sr, and trivalent chromium, an oxidizing substance source, and an oxyacid or oxyacid salt of phosphorus or its anhydride.
In each of the above-described patents, an oxidizing agent is included as an essential ingredient in the hexavalent chromium-free solution. It is known in the art that the presence of oxidizing agents may lead to some conversion of trivalent chromium to hexavalent chromium during the formation of the conversion coating. The presence of this hexavalent chromium may then lead to some or all of the previously described problems. Furthermore, the inclusion of an oxidizing agent may add additional costs and steps to the formation of the conversion coatings.
Therefore, the development of a hexavalent chromium-free conversion coating that provides similar attributes to traditional hexavalent chromium systems is desirable. Such a hexavalent chromium-free coating is preferably substantially impervious to moisture and able to withstand a salt spray test for up to about 12 to 24 hours. Furthermore, the coating should simultaneously enhance the appearance of the substrate by imparting an attractive, brightly polished finish.
The present invention provides an aesthetically pleasing, and corrosion inhibiting, hexavalent chromium-free, trivalent chromium conversion coating.